Recently, along development of compact information equipment, electronic components to be mounted have been downsized rapidly. A ball grid alley (hereinafter referred to as “BGA”) having electrodes at its rear surface is applied to such electronic components in order to cope with a narrowed connection terminal and a reduced mounting area because of the downsizing requirement.
As the electronic components to which the BGA is applied, for example, a semiconductor package is exemplified. In the semiconductor package, semiconductor chips having electrodes are sealed with resin. Solder bumps are formed on the electrodes of the semiconductor chips. This solder bump is formed by joining a solder ball to an electrode of the semiconductor chip. The semiconductor package to which the BGA is applied is mounted on a printed circuit board by joining the solder bump melted by the heating to a conductive land of the printed circuit board. Additionally, a three-dimensional high-density mounting structure has been studied by stacking up the semiconductor packages in a height direction in order to meet the further high-density mounting requirement.
However, in a case that the BGA is applied to the semiconductor package for the three-dimensional high-density mounting structure, the solder ball may be crushed by semiconductor package's weight. If such an accident happens, an appropriate space between the substrates cannot be maintained.
Accordingly, a solder bump has been considered, where a Cu ball or a Cu core ball in which solder plating is covered on a surface of the Cu ball, is electrically joined on an electrode of an electronic component by using solder paste. The solder bump formed by using the Cu ball or the Cu core ball can support a semiconductor package by the Cu ball, which is not melted at a melting temperature of the solder, even if the solder bump receives the weight of the semiconductor package when the electronic components are mounted on the printed circuit board. Therefore, the solder bump is not crushed by the semiconductor package's weight. As related art, for example, Patent Document 1 is exemplified.
A Cu ball is formed by melting a small piece of Cu material with the heating to make it a ball. Patent Document 2 discloses a method of forming a Cu ball by spherically coagulating Cu metal powder after it is cast into thermal plasma and melted. It is common for the Cu balls formed by such methods to be cooled rapidly to room temperature or cooling gas temperature, in terms of expediting an amount of production or a production rate.